The present invention generally relates to a method of manufacturing semiconductor chips and the semiconductor chips thus obtained. More particularly, the present invention relates to thinning of a semiconductor device wafer and semiconductor chips manufactured by using the thinned semiconductor device wafer.
In recent years, high density and integration of devices are continuously made in the field of compound semiconductors. As mobile communication equipment becomes more compact and lightweight, the devices become progressively fine. As the chip area becomes smaller, the thermal resistance of the device becomes increasingly high. It is indispensable to reduce the thermal resistance of the devices to allow realization of high-performance and high-reliability devices. For that purpose, various designs for improving heat radiation effect have been adopted. The most effective method of decreasing the thermal resistance of the device is to form a pattern of integrated circuit elements on a semiconductor wafer and then thin the semiconductor wafer to increase heat radiation to the rear surface of the semiconductor wafer.
FIG. 6 shows a conventional method of thinning a semiconductor device wafer 1. To thin the semiconductor device wafer 1, a protection resist 2 is applied to a surface of the semiconductor device wafer 1. Then the protection resist-applied surface of the semiconductor device wafer 1 is fixedly bonded to a vacuum sucking pedestal 4 with electron wax 3. Thereafter the vacuum sucking pedestal 4 is sucked to an abrading base 5 under vacuum and thinning of the wafer is performed. To thin the semiconductor device wafer 1, a method of grinding the semiconductor device wafer 1 with a grindstone or a method of polishing or abrading it with an abrasive material may be used. In the case where the semiconductor device wafer 1 is ground, a grindstone having a particle diameter of not less than 40 μm is typically used, and the grinding process is performed under flowing water. In the case where the semiconductor device wafer 1 is polished or abraded, a diamond oil abrasive material 6 having a particle diameter in the range of 3 μm to 9 μm and an oil lubricant are used. Reference numeral 7 denotes an abrading platen.
The particle diameter of the grindstone is larger than that of an abrasive material. Therefore when the grinding method is used, the thinning speed is higher, but the grindstone tends to give damage such as deep flaws to the surface of the semiconductor device wafer 1. Thus when the semiconductor device wafer 1 is ground thinly, it is liable to crack. Grinding is capable of thinning the semiconductor device wafer 1 to a thickness of about 150 μm. To thin the semiconductor device wafer 1 to a thickness of less than 150 μm, abrasion is normally used. In the abrasion, an abrasive material having a smaller particle diameter than the grindstone is used, and an oil abrasive material smooth and giving a low extent of damage to the surface of the semiconductor device wafer 1 is used.
Attention is particularly necessary to thinly abrade a GaAs substrate, which is more crackable than a Si substrate. For example, when the GaAs substrate is used for the semiconductor device wafer 1, the grinding, which allows the mass-production, is used to thin it to a thickness of about 150 μm or less. However, as described above, the grinding cannot be used to thin the semiconductor device wafer 1 to a thickness of about 100 μm or less.
When the GaAs substrate is desired to be thinned to a thickness of 100 μm or less, abrasion is used. However, when the GaAs substrate is abraded with an aqueous abrasive material, the GaAs substrate is much damaged. Thus an oil abrasive material giving small damage to the GaAs substrate is used normally.
However, even in the case where an oil abrasive material is used, a GaAs semiconductor device wafer having a dimension of not less than three inches is liable to crack in the stages of handling, cleaning, feeding, and/or mounting. Thus, disadvantageously, there was a limit (80 μm) in thinning the semiconductor device wafer by the conventional method.
In both the grinding method and the abrading method, it is necessary to bond the semiconductor device wafer 1 to the vacuum sucking pedestal 4 with the electron wax 3, separate the semiconductor device wafer 1 from the vacuum sucking pedestal 4 after it is thinned, and wash out the wax and the abrasive material. Thus it takes much time and costs high to thin the semiconductor device wafer 1. In particular, that is the case with the oil abrasive material. A high-viscosity oil lubricating material is typically used as the oil abrasive material. Consequently, the oil of the lubricating material remains in irregularities of an abraded surface of the semiconductor device wafer 1. It is difficult to completely remove remaining oil by means of an organic detergent. Therefore it is necessary to physically remove the oil by performing a mirror finishing process. However, the mirror finishing process is very inefficient in mass production.
When the oil remains on the abraded surface (rear surface) of the semiconductor device wafer 1, the oil adversely affects the adhesiveness of a metal layer to be plated on the rear surface of the semiconductor device wafer 1 in a later process. Consequently the metal layer is liable to be separated from the semiconductor device wafer 1.